What happend is: The seller removed the "Buy it now" button, he might have realized that the scope is worth more than $650. It's still on auction, but might not be a bargain any more (depending how high the offers go until the auction is finished).
What normally happens with eBay listings with Buy It Now is that once someone makes a bid on the item, the Buy It Now option is removed as the item is now on auction and no longer for sale at fixed price.